Topically applied antiperspirant compositions are in widespread use throughout much of the world, in order to enable their users to avoid or minimise wet patches on their skin, especially in axillary regions. Antiperspirant formulations have been applied using a range of different applicators depending on the individual preferences of consumers, including aerosols, roll-ons, pump sprays, sticks and so-called mushroom applicators which are used to apply cream formulations. In some parts of the world, sticks are especially popular. The term stick traditionally indicates a bar of material with a solid appearance which was usually housed within a dispensing container and which retains its structural integrity and shape whilst being applied. When a portion of the stick is drawn across the skin surface a film of the stick composition is transferred to the skin surface. Although the stick has the appearance of a solid article capable of retaining its own shape for a period of time, the material usually has a structured liquid phase so that a film of the composition is readily transferred from the stick to another surface upon contact. More recently, the term "stick" has been loosely applied to soft solids, which have a solid form during storage, but which flow under mild pressure or shear, so that in use they can be extruded through an aperture or apertures onto a dispensing surface.
Antiperspirant sticks have been made with a variety of different types of composition. Suspension sticks go contain a particulate antiperspirant active suspended in a structured carrier material which may well be anhydrous. Solution sticks have the antiperspirant active dissolved in a structured carrier which is polar and may be aqueous or may be based on a non-aqueous polar solvent such as ethanol. A third form of stick is an emulsion of two phases where the continuous phase is structured so that the composition is able to sustain its own shape, the antiperspirant active being dissolved in the more polar of the two phases present. In some emulsion sticks the antiperspirant active is dissolved in an aqueous disperse phase so that the composition can be classified as a water-in-oil emulsion. The classification into suspension, emulsion and solution types can be applied to both firm and soft solid compositions.
Although a number of documents have disclosed emulsion sticks, the marketplace has avoided them. Antiperspirant stick products currently on the market are either suspension sticks in which the antiperspirant active is dispersed in particulate form or are solution sticks in which the antiperspirant active is dissolved in a structured single phase which tends to feel tacky or draggy when applied to the skin.
There is substantial literature on the structuring or thickening of antiperspirant compositions which is frequently accomplished using some form of thickening agent as part of the composition.
Structuring a stick by the incorporation of a thickening polymer has been disclosed in a number of documents including U.S. Pat. No. 5,500,209, U.S. Pat. No. 5,783,657 and WO 99/06473.
A class of polymers which has been used in this way is the polyamides. Compositions in this way tend to have poor sensory properties experienced by the user, notably a sticky or rubbery feel as they are applied.
It has been common practice for sticks to be structured by incorporating fatty alcohol and/wax into the composition. Sticks which are structured with fatty alcohol or wax tend to leave visible white deposits on application to human skin. These deposits can also transfer onto clothing when it comes into contact with the skin and the wearer can, for example, find white marks on the armhole of a sleeveless garment.
There have been some disclosures of antiperspirant stick compositions where structuring to a shape-sustaining stick has been accomplished without using a fatty alcohol or wax. Amongst these disclosures there has sometimes been recognition that white deposits are avoided.
Antiperspirant stick compositions are known which do not contain a structuring agent and which are translucent or transparent. U.S. Pat. No. 4,948,578 discloses transparent antiperspirant sticks formed as emulsions where the aqueous phase contains a large proportion of nonionic surfactant as well as water and dissolved antiperspirant active.
Compositions with high viscosity but without gelling or structuring agent are also disclosed in U.S. Pat. No. 4,673,570 and U.S. Pat. No. 5,587,153. These compositions are believed to take the form of creams or soft gels which are extruded from a suitable package by the user.
In some compositions structuring has been achieved by the incorporation of a structurant (also referred to as a gellant or gelling agent) which causes the liquid to gel upon cooling from an elevated temperature. Gel formation takes place as an exothermic event within a temperature range referred to as the gel point; upon reheating, melting of the gel takes place as an endothermic event within a temperature range. Such gels can be disrupted by shearing and do not recover their structure for a long time, if at all unless remelted, although a small partial recovery may be observed.
U.S. Pat. No. 5,455,026 (Bahr) discloses and exemplifies gels of silicone oil containing 12-hydroxystearic acid--frequently in rather small amounts--together with a particulate antiperspirant active. A further possibility which is mentioned but never exemplified is that the antiperspirant active may be present in solution in an organic solvent. Ethanol and propylene glycol are mentioned as possibilities. This document states that clear gels may be obtained by matching the refractive index of the antiperspirant to that of the silicone oil.
Fatty acyl amino acid amides, 12-hydroxy stearic acid and dibenzylidene sorbitol are all examples of compounds which are able to gel and hence structure at least some hydrophobic, water-immiscible organic liquids, although dibenzylidene sorbitol will not structure an organic liquid if an acidic aqueous phase is present, because it will be hydrolysed rapidly. We now believe that they function by forming a network of fibres, which appear to be branched or interconnected, and which extend throughout the liquid and thereby give it rigidity. When the gel melts these fibres dissolve in the liquid.
Some ingredient materials which are used in sticks have accompanying disadvantages which are a complicating factor in the formulation of compositions. If a polar organic solvent is volatile, like ethanol, the stick gives a cooling sensation when applied. Some cooling may be desired but too much may prove unacceptable to consumers. Polar but less volatile solvents such as water-immiscible diols tend to make a stick feel tacky when touched and hence give a sensation of stickiness and drag when applied to skin.